1. Field of the Invention
The present invention relates to a display device having an electric signal processing circuit including thin film semiconductor elements and which drives an output device for outputting other signals than a video signal, and more specifically to a display device in which an audio signal processing circuit for driving a speaker is integrally formed on a substrate. In this specification, the term speaker includes a transducer which outputs audio by vibrating members of a display device such as a glass substrate, a plastic substrate, or a touch panel, and refers to devices in general that convert electric signals into audio.
2. Description of the Related Art
The advance of communication technologies has made cellular phones popular in recent years, and now more information communication including transmission of motion pictures is expected. Meanwhile, personal computers have become light-weight, prompting manufacture of mobile type products. Also, information terminals called PDA which originate from electronic organizers are being mass-produced and gaining popularity. Most of such portable information appliances have flat panel displays owing to development of display devices.
Among active matrix type display devices, ones that use low temperature polysilicon thin film transistors (hereinafter a thin film transistor is referred to as a TFT) has recently begun to be commercialized. Low temperature polysilicon TFTs make it possible to form not only a pixel portion but also a signal line driving circuit integrally with the driving circuit placed in the periphery of the pixel portion. Therefore, low temperature polysilicon TFTs are helpful in downsizing display devices and increasing a definition of display devices, and their further propagation is predicted.
In addition to the image display function, other output functions, an audio output function in particular, are requested for portable information appliance. With audio, image display can be more effective and accordingly more fully appreciated.
Usual audio output devices output audio by converting electric signals into audio through cone speakers or the like. Corn speakers take up a large area inside portable information appliance and therefore present an obstacle to reduction in size and weight of portable information appliance.
FIGS. 2A and 2B are profile diagrams showing a display device and its periphery in conventional portable information appliance with an audio output function. The display device has a substrate 209 on which a pixel portion 204, a source signal line driving circuit 202, and a gate signal line driving circuit 203 are formed integrally. FIG. 2A is a top view of the display device and FIG. 2B is a lateral view of the display device. A source signal line driving circuit 202, a gate signal line driving circuit 203 and a pixel portion 204 are omitted in FIG. 2B for simple display. Attached to the display device are a cone speaker 207, an FPC 205, an opposite substrate 208, and an audio signal processing circuit 210 mounted on a printed board 206.
The cone speaker 207 is unfit for reduction in size and weight of the portable information appliance due to its large size. For that reason, flat speakers as the one shown in FIGS. 3A and 3B are being developed. FIGS. 3A and 3B are profile diagrams of a display device and its periphery in portable information appliance 301 with a flat speaker. The display device has a substrate 309 on which a pixel portion 304, a source signal line driving circuit 302, and a gate signal line driving circuit 303 are formed integrally. FIG. 3A is a top view of the display device and FIG. 3B is a lateral view of the display device. A source signal line driving circuit 302, a gate signal line driving circuit 303 and a pixel portion 304 are omitted in FIG. 3B for simple display. Attached to the display device are a flat speaker 306, FPCs 305 and 308, an opposite substrate 310, and an audio signal processing circuit 307 mounted on a printed board 311.
The flat speaker is similar to conventional speakers in that electric signals are converted into vibrations to output audio but is different in that a glass substrate, a plastic substrate, a touch panel, or the like of the display device or others is vibrated instead of the cone. With such a flat speaker, the portable information appliance can have a smaller size and lighter weight than a conventional one that uses a cone speaker.
FIG. 17 shows an example of a flat speaker. In this example, a touch panel 1702, a lower substrate 1705, an adhesive 1706, and a liquid crystal panel 1704 are set in a chassis 1701 and vibrated by a speaker 1703 to output audio. (See. Nikkei Electronics, Aug. 26, 2002, p. 52)
Although flat speakers are very effective means in reducing the size and weight of portable information appliance as described above, there is the following problem to be solved:
As shown in FIGS. 3A and 3B, the audio signal processing circuit 307 for driving the flat speaker 306 has the printed board 311 placed outside the display device and an LSI is mounted on the printed board 311 similarly to conventional portable information appliance. This is still insufficient for reduction in size and weight of portable information appliance.